The present invention relates to a novel epoxy resin composition or, more particularly, to a heat-curable flame-retardant epoxy resin composition suitable for case potting of film capacitors.
It is an established technology that a film capacitor roll encased in a plastic casing is impregnated with a heat-curable flowable resin composition such as an epoxy resin-based composition followed by heating to effect curing of the resin composition so as to complete a film capacitor with the plastic casing to serve as a protective envelope while it is important that the heat-curable resin composition is flame retardant as highly as possible as an inherent requirement for electric parts.
One of flame-retardant epoxy resin compositions of a typical formulation in the prior art and suitable for such an application comprises an epoxy resin, polycarboxylic acid anhydride as the curing agent, red phosphorus powder and powder of hydrated alumina as is disclosed in Japanese Patent Kokai 54-93044, 59-98123, 62-27420, 64-29463 and elsewhere. It is taught in Japanese Patent Kokai 60-115620, 61-176626, 61-276816, 62-7720 and 1-198658 and in Japanese Patent Publication 5-19806 that a flowable epoxy resin composition used for impregnation and encapsulation of various kinds of electronic parts such as flyback transformers, coils, capacitors and diodes can be imparted with improved impregnability and decrease in sedimentation or separation of particulate ingredients during storage of the liquid composition when the powder of hydrated alumina contained therein has a specific particle size distribution.
The above described improvement relative to the impregnating behavior and settling of the particulate ingredients such as hydrated alumina powders accomplished in the prior art epoxy resin compositions is still not very satisfactory for a flame-retardant epoxy resin-based composition used for case potting of film capacitors because the epoxy resin composition for case potting of film capacitors is required to exhibit, in addition to the requirement for complete absence of settling of particulate ingredients, excellent behavior of impregnation not only to completely spread over the surface of the capacitor roll but also to completely and uniformly fill up any smallest interstices such as the interlayer spaces of the film layers and voids in the plastic casing to the very corners.